The present invention relates in general to integrated circuits, and in particular to various embodiments for a resistive pull-up device on an integrated circuit input/output (I/O) terminal.
An I/O pin of an integrated circuit may be connected to an external bus that is tri-statable. Under the tri-state condition, the voltage level at such an I/O pin may be floating at levels that may cause excessive current dissipation by the I/O circuitry inside the integrated circuit. Thus, in certain applications, I/O pins in an integrated circuit are required to have a built-in resistive pull-up or pull-down device to avoid the high-current tri-state conditions. Resistive pull-up or pull-down devices are also used as programmable resistors in open drain (or open collector) applications where wired OR logic is implemented.
A common prior art approach to implementing a built-in pull-up device has been to add a diode-connected n-channel transistor that connects the I/O pin to the positive power supply rail Vcc. An example of this prior art approach is shown in FIG. 1. When the external node is tri-stated, n-channel transistor 100 pulls the I/O node up to Vcc-Vtn, where Vtn is the threshold voltage of the n-channel transistor. Transistor 100 is designed to be weak enough and easily over-driven by internal or external logic. One advantage of using an n-channel transistor is that when the pin voltage goes above Vcc, the diode-connected n-channel transistor turns off and ensures that no current flows back into the power supply line. There is a drawback, however, in that the circuit of FIG. 1 might still cause appreciable amounts of standby (DC) current Icc to be dissipated when the pin is tri-stated.
When the pin is tri-stated, diode-connected n-channel transistor 100 pulls this node up but not all the way to Vcc. As mentioned earlier, the pin voltage is pulled up to Vcc-Vtn, where Vtn is the threshold voltage of n-channel transistor 100. It is very common for the I/O pin to drive a TTL input buffer 102 inside the integrated circuit as shown in FIG. 1. A TTL buffer typically includes an inverter with the sizes of p-channel transistor 104 and n-channel transistor 106 adjusted to have a trip point at about 1.4 volts. One function of the pull-up device is to ensure that the input buffer is in a known state when external or internal drivers aren't driving the I/O pin. In the case of the circuit of the FIG. 1, the pull-up transistor is intended to operate to turn off p-channel transistor 104 and turn on n-channel transistor 106 for a logic low at the output of buffer 102. However, if the threshold voltage Vtn of n-channel pull-up transistor 100 is greater than the absolute magnitude of the threshold voltage .vertline.Vtp.vertline. of p-channel transistor 104, p-channel transistor 104 would have a source-to-gate voltage larger than its threshold voltage, and would thus be slightly turned on and conductive. This problem is exacerbated by the fact that the threshold voltage Vtn for n-channel transistor 100 may be increased by body effect. With Vcc-Vtn at its gate terminal, p-channel transistor 104 is turned on even harder during the switching on of n-channel transistor 106 due to body effect. The resulting crowbar current can be in the tens of microamps, which can be significant in lower power integrated circuits.
There is therefore a need for an improved pull-up device for I/O pins, that minimizes current dissipation by the circuit.